This application claims the priority benefit of Taiwan application Ser. No. 89118303, filed Sep. 18, 2000.
1. Field of the Invention
The present invention relates to a photolithography process in a semiconductor process. More particularly, the present invention relates to a process for increasing a line width widow in a semiconductor process.
2. Description of the Related Art
With the increasingly higher demand for the level of circuit integration, circuit design is compelled to move forward in the direction of continuous reduction in the size of the components of an entire circuit. In the whole semiconductor process, the most decisive step is photolithography. Anything is related to metal-oxide-semiconductor (MOS) component structure, for example, the design of each layer of film and the areas with dopants, is decided by the photolithography step.
In addition, whether the component integration of the whole semiconductor industry can continue to advance to a line width less than 0.13 xcexcm will also be decided by the technological development of the photolithography process. In order to meet this demand, processes for increasing photomask resolution, such as the process of optical proximity correction (OPC), are put forward constantly.
The object of OPC is to eliminate the phenomenon of critical dimension (CD) bias caused by proximity effect. The proximity effect occurs when a light beam projects on the wafer through the photomask pattern. On the one hand, because the light beam produces a scattering phenomenon, the light beam is enlarged; on the other hand, the light beam passes through the photoresist layer on the surface of the wafer and reflects from the semiconductor base of the wafer, and thus a phenomenon of interference is produced. Therefore, there are repeated exposures that change the actual amount of exposure on the photoresist layer. The smaller the critical dimension of the process is, the more obvious this phenomenon becomes, especially when the critical dimension is close to the wavelength of the light source.
When the pattern that is needed is an iso-line, the required exposure condition is different from the exposure condition for a dense-line pattern because there is no other pattern near the iso-line. Even for a dense-line pattern, if the line width duty ratio is different, the exposure condition required is different accordingly. Therefore, in processes under 0.13 xcexcm, it is very difficult to compensate for the proximity bias produced by different patterns in order to achieve satisfactory development results.
Scattering bar is a process used for correcting and reducing proximity effect in the photolithography process. Bar-like patterns are formed along the periphery of the pattern on the photomask. When exposure is conducted, the light wave passes the scattering bar and scatters so that the proximity effect on the original design is eliminated.
Scattering bars are often used to correct the proximity effect on iso-line pattern. Usually, scattering bars are added on both sides of the iso-line pattern so as to make the exposure condition closer to that of the dense line. In this way, the process window for exposure conditions is enlarged. However, if the scattering bars are too wide, they cause positive photoresist to remain in the corresponding positions on the base. On the other hand, if the scattering bars are not wide enough, they do not sufficiently eliminate the proximity effect. Therefore, iso-line patterns often cause much limitation to the scope of exposure conditions.
One of the objects of the present invention is to provide a process for increasing a line width window in semiconductor process in order to solve the problem that occurs when an iso-line pattern and a dense line pattern undergo exposure at the same time.
Another object of the present invention is to provide a process for increasing a line width window, wherein scattering bars of sufficient size are added on both sides of the iso-line pattern to undergo the first exposure. Then, the corresponding parts of the scattering bars undergo a second exposure with a second photomask in order to eliminate the remaining photoresist at the positions corresponding to the scattering bars.
Based on the above described objects of the invention, a process for increasing the line width window of semiconductor process is provided, which process is suitable for increasing the photolithography line width widow at the time of the exposure of an iso-line pattern under 0.13 xcexcm. This process includes: first forming a positive photoresist layer on the base and then using the first photomask to conduct the first exposure step on the positive photoresist layer. The first photomask is designed to have at least one opaque main line. On each of the two sides of the main line, there is an opaque scattering bar. The width of the two scattering bars is greater than one third of the wavelength of the light source being used, and less than the width of the main line. The second photomask is used to conduct the second exposure step on the positive photoresist layer. The second photomask is designed to have at least two iso-lines that are pervious to light, and each of the two iso-lines is located in one of the two positions corresponding to the two scattering bars of the first photomask design. The width of each iso-line is greater than that of the corresponding scattering bar and the distance from each edge of the iso-line to each edge of the corresponding scattering bar is greater than about 60 nm.
As is described above, the present invention uses scattering bars of sufficient size to eliminate the proximity effect during the main line exposure. It uses another exposure to solve the problem of photoresist remaining caused by the scattering bars. Therefore, the present invention can increase the line width widow and enables the process of fabrication to enter more easily into process under 0.13 xcexcm.